The specific aim of this application is to study the effect of physiological tumor oxygen (O2) concentrations (0.1-10%) vs. traditional use of 20% O2 as related to the acute and chronic effects of O2 on (a) tumor cells in tissue culture (b) solid tumor colony grown (c) cell cycle distribution and time (d) the interpretation of in-vitro drug and radiation cytotoxicity. Methods include traditional tissue culture, labeled precursor uptake and chroimosome studies complemented by the more recent (1) in-vitro and in-vivo clonogenic assays of cell survival and (2) use of flow cytometry for monitoring cell cycle and ploidy changes. Our preliminary studies suggest not only the beneficial effects of low O2 concentration on solid tumor colony growth but also the differential cytoxicity data that are obtained when cells are cultured after drug treatment in 5 vs 20% O2. Current in-vitro testing using clonogenic assays of human tumors for predicting responses clinically is limited technically in the number of drug assays which can be carried out because of poor colony growth. It is hoped, that these studies would clarify the role of physiological tumor o2 concentration on (1) tumor colony growtha and help to define an optimum concentration which mayh be clinially useful and (2) the interpretation of drug and radiation cytotoxicity data as they are important for accurate ranking of drugs for trials clinically and experimentally.